1. Field of the Invention
The present invention relates to an optical transmission system used for optical communication, CATV, optical measurement, mobile communication and the like, and also relates to a voltage controlled oscillator used for the optical transmission system.
2. Description of the Related Art
In recent years, in video monitoring systems, CATV, subscriber systems, mobile communication systems and the like, optical transmission of multi-channel video and audio signals or data has gotten into practical use by making full use of the low loss and wideband characteristics of optical fibers. In these systems, multi-channel signals are electrically multiplexed by a plurality of subcarriers having different frequencies to obtain AM signals, and the AM signals are converted into an optical signal by directly modulating a semiconductor laser or the like and transmitted via an optical fiber. The modem for the video optical transmission of the AM signals is simple in structure and low in cost; however, the receiver requires high optical input power to obtain a C/N ratio (carrier-to-noise ratio) for ensuring desired video quality. In addition, in mobile communication, the intensity levels of audio and data signals to be transmitted change significantly depending on the movement of terminals; therefore, a high dynamic range is required to cope with the signal change. Furthermore, transmission is apt to be adversely affected by distortion due to reflected waves during optical conversion at the semiconductor laser or in the middle of transmission via the optical fiber, thereby causing a problem. Moreover, an amplifier having excellent linearity is required for an AM signal amplifier.
A system has been proposed wherein AM signals multiplexed by subcarriers so as to enhance resistance against distortion and noise are converted into an FM signal as a whole and transmitted optically. The modulation index is required to be increased to obtain a desired C/N ratio in optical transmission. Accordingly, a system has also been proposed wherein the frequency of a semiconductor laser is modulated directly to obtain an FM signal having a high modulation index.
FIG. 6 is a view showing the configuration of a conventional FM signal optical transmission system. The AM/FM converter 62 of a transmitter 61 directly modulates a semiconductor laser 41 by using multi-channel AM video signals and outputs an optical frequency-modulated signal. Optical amplitude modulation and oscillation frequency modulation are carried out simultaneously by directly modulating the semiconductor laser 41 by using AM signals 30. By a multiplexer 43, this optical signal is multiplexed with the light of a local oscillation light source 42 having an oscillation frequency slightly different from that of the optical signal, input to a photodiode 44, and optically heterodyne-detected, thereby obtaining an FM-modulated signal having a wideband (1 to 6 GH for example) as a beat signal for the two lasers. An electric/optic converter 63 directly modulates a transmission semiconductor laser 69 by using a semiconductor laser drive amplifier 68 to convert the FM-modulated signal into an optical signal, and the optical signal is transmitted via an optical fiber 72 (Patent Publication No. 2700622 for example).
Hereafter, the optical signal is amplified by an optical amplifier or the like, and transmitted to individual receivers via optical branching devices. Only one receiver 73 is shown here. At the receiver 73, an optic/electric converter 75 converts the optical signal into an electric signal by using a photo-detector 76, amplifies the electric signal by using a preamplifier 77, and then an FM demodulator 74 demodulates the electric signal into the original AM signals 31. This FM demodulator 74 is a delay-type demodulation circuit comprising high-speed logic ICs 51 and 53 (AND gates for example), a delay device 52 and a low-pass filter 54 via a limiter amplifier 50, and is capable of carrying out wideband demodulation.
However, in the above-mentioned system, when AM signals are converted into an FM signal, the phase noise of the semiconductor laser is added to the FM signal, thereby lowering the C/N ratio. Therefore, even if the intensity of optical reception is raised, it is impossible to improve the sensitivity so as to have a constant C/N ratio value or more. To obtain a desired C/N ratio, a semiconductor laser is required to have an about one-tenth line width of that of a conventional semiconductor laser, and a semiconductor laser having an external oscillator structure or the like has been used. However, the semiconductor itself is expensive, and a plurality of semiconductors must be used. In order to carry out optical heterodyne detection with low noise, the temperature and drive current of the semiconductor laser must be controlled and stabilized highly accurately, whereby the configuration becomes complicated and the whole system becomes high in cost. In addition, this causes a problem of taking a longer time until the whole system operates stably after power on.
Furthermore, it is considered that AM signals are converted directly into an electrical FM signal; however, if the modulation index at the FM demodulator is increased (the degree of modulation xe2x89xa710%), distortion occurs at the FM modulator, and this distortion deteriorates the quality of the signal, whereby it is impossible to carry out satisfactory optical transmission.
Moreover, the demodulator also uses a plurality of expensive high-speed logic ICs; therefore, this causes a problem in practical utility when it is assumed to be used for subscriber systems.
In consideration of these problems in the conventional FM signal optical transmission system, the present invention is intended to provide an FM signal optical transmission system and an FM signal optical transmitter having a simple configuration, low distortion and excellent reception sensitivity, and also to provide a voltage controlled oscillator at low cost in order to attain the FM signal optical transmission system and the FM signal optical transmitter.
The 1st invention of the present invention (corresponding to claim 1) is a voltage controlled oscillator for controlling the oscillation frequency of an output signal depending on an input voltage, being characterized to receive a modulated voltage and output an FM signal having a band including a fundamental oscillation frequency and plural higher-order harmonic components of said fundamental oscillation frequency.
The 2nd invention of the present invention (corresponding to claim 2) is a voltage controlled oscillator in accordance with said 1st invention, wherein a frequency band including only the predetermined higher-order harmonic component is filtered at the output stage.
The 3rd invention of the present invention (corresponding to claim 3) is an FM signal optical transmitter comprising a signal processor for outputting an FM signal having a band including a fundamental oscillation frequency and plural higher-order harmonic components of said fundamental oscillation frequency, a band-pass filter for taking out only the predetermined-order harmonic signal component from said FM signal output from said signal processor, a frequency converter for shifting the taken-out harmonic signal component to the lower frequency side or the higher frequency side, and an electric/optic converter for converting the output signal of said frequency converter into an optical signal.
The 4th invention of the present invention (corresponding to claim 4) is an FM signal optical transmitter comprising a signal processor for outputting an FM signal having a band including a fundamental oscillation frequency and plural higher-order harmonic components of said fundamental oscillation frequency, a band-pass filter for taking out only the predetermined-order harmonic signal component from said FM signal output from said signal processor, a frequency multiplier that directly multiplies the taken-out harmonic signal component or shifts said harmonic signal component to the lower frequency side or the higher frequency side and then multiplies the shifted signal, and an electric/optic converter that directly converts the frequency-multiplied signal into an optical signal or shifts said frequency-multiplied signal to the lower frequency side or the higher frequency side and then converts the shifted signal into an optical signal, wherein
the modulation degree of said frequency-multiplied signal is raised to a predetermined modulation degree at the time of multiplication.
The 5th invention of the present invention (corresponding to claim 5) is an FM signal optical transmitter in accordance with said 3rd or 4th inventions, wherein said signal processor for outputting said FM signal is a voltage controlled oscillator in accordance with said 1st or 2nd inventions, and said modulated voltage is formed of plural subcarrier-multiplexed signals.
The 6th invention of the present invention (corresponding to claim 6) is an FM signal optical transmitter in accordance with said 3rd invention, wherein a predetermined harmonic carrier wave component is extracted from some harmonic components at the output of said signal processor, and the extracted harmonic carrier wave component is used as a reference frequency source required when frequency conversion is carried by said frequency converter.
The 7th invention of the present invention (corresponding to claim 7) is an FM signal optical transmitter in accordance with said 4th invention, wherein said frequency shifting is carried out by a frequency converter, a predetermined harmonic carrier wave component is extracted from some harmonic components at the output of said signal processor, and the extracted harmonic carrier wave component is used as a reference frequency source required when frequency conversion is carried by said frequency converter.
The 8th invention of the present invention (corresponding to claim 8) is an FM signal optical receiver comprising an optic/electric converter that receives an optical signal transmitted from said FM signal optical transmitter in accordance with any one of said 3rd to 7th inventions and converts said optical signal into an electric signal, and an FM demodulator for demodulating an FM signal converted into said electric signal.
The 9th invention of the present invention (corresponding to claim 9) is an FM signal optical transmission system comprising said FM signal optical transmitter in accordance with any one of said 3rd to 7th inventions, an optic/electric converter that receives an optical signal transmitted from said FM signal optical transmitter and converts said optical signal into an electric signal, and an FM demodulator for demodulating an FM signal converted into said electric signal.